EP 0810368 A2 (JP-9-317619 A) discloses an ignition device of this type, which has a capacitor that stores the current flowing in a direction between an ignition plug of an internal combustion engine and the ground when the plug discharges with sparks. After the ignition plug discharges, the capacitor discharges with a current flowing in the opposite direction. The ignition device compares this current with a preset threshold. By taking advantage of the property of the ion current flowing in the foregoing opposite direction through the ions produced when a mixture burns normally in the combustion chamber of the internal combustion engine, the ignition device determines if a misfire has occurred on the basis whether this current is not smaller than the threshold.
Generally, in an internal combustion engine, additives etc. contained in the fuel and/or lubricant may carbonate and stick to the ignition plug, causing the plug to smolder. When the ignition plug smolders, the insulation quality between its electrodes decreases. When an ion current flows, the decrease in insulation quality varies the manner in which the current sensed at a point downstream from the ignition plug flows.
The ignition device detects the smoldering degree from the current flowing after the ion current flows. The threshold is set according to the detected smoldering degree. As a result, even when a smolder occurs, it is possible to properly determine if a misfire has occurred.
When the smoldering degree rises, the internal combustion engine may tend to operate improperly. However, even when the internal combustion engine gets into such a situation, the ignition device merely determines that the situation is a misfire. In the ignition device, in order to implement a function of current detection for grasping the smoldering degree and a function for varying the threshold on the basis of the detected current, the scale of the circuits of the ignition device and the electronic control unit that controls the output from the internal combustion engine increases. The increase cannot be neglected.